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Many (most? all?) Spanish words containing the letter h come from corresponding Latin words containing the letter f. Through what process did /f/ get softened to /h/? During what time period did this change occur?

A few examples include:

hablar from deponent Latin verb for, to speak, whose future and past are fābor and fābar. (Funny that it's famously regular in Spanish)

@Alenanno: I agree but it didn't seem to be resolved there whether questions about a single language belonged there, especially when the language had its own site.
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hippietrailDec 2 '11 at 16:25

A side comment: Most, but not all apparences of H come from Latin, many come from Arabic, as Almohada (pillow), Albahaca (Basil), pretty much those beginning with the Arabic definite article al
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PetruzaDec 31 '11 at 2:35

And of course there are other loan words with an h. An obvious example: hotcakes (often, but not always, pronounced as with an English 'h' sound)
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Flimzy♦Feb 24 '12 at 22:28

The disappearance of initial f in many words that in Latin have this sound, and, presumably, the so-called betacism, is probably caused by the influence of Basque or Iberian (note that the aspiration of /h/ is also found in the Gascon dialect that would have had the same Basque substrate).

Some scholars believe that one language that served as a substrate for Ibero-Romance language was Basque, which possibly contributed to the change from /f/ to /h/ at the beginning of words in Spanish (Latin farina became "harina"), and words like "izquierda" (Basque ezkerra).